Structure And Properties Of Three-dimensional Needle-punched Carbon Felt Reinforced Resin-based Carbon Composites

The three-dimensional(3D)needle-punched carbon felt reinforced resin-based carbon composites used carbon fiber needle-punched preform as reinforcements and resin-based carbon as matrix.The preparation of C/C composites included two processes,one is structural design and fabrication of preform,and the pore filling process in the preform by recycled impregnation and carbonization.As one of the most important needle-punching parameters,the needle-punched density affects the fiber contents in thickness direction and the pore distribution between fibers in needle-punched preforms.On the one hand,the fiber contents in the thickness direction of preform have an important influence on the inter-laminar bonding and mechanical properties of resin-based carbon composites.On the other hand,pores in preform will provide the impregnation channel for resin and subsequently control the filling state of resin-based carbon,which ultimately affects the internal porosity and mechanical properties of resin-carbon composites.Therefore,in this work,the effect of changing the needle-punched density on pore structure of preforms was studied.The evolution of pores during the impregnation and carbonization was investigated,and the pore evolution model was established by fractal method.The failure modes of resin-based carbon composites were summarized,and the effect of pore structure on the mechanical properties was analyzed.The study of preform structure,densification process and mechanical properties can provide experimental and theoretical basis for building an integrated system of preform structure-preparation-performance.In this work,four kinds of preforms with different needle-punched densities reinforcement resin-based carbon composites were prepared by liquid phase infiltration process.The effects of needle-punched densities with 20,30,40 and 50 P/cm~2 on preform structure,densification process and mechanical properties of resin-based carbon composites were studied.The main conclusions are as follows.(1)The arrangement of carbon fiber bundles in the weft-less layer is highly oriented.The fibers in the mesh layer are arranged in a disorderly manner and the structure is relatively loose.The large pores inside preforms are mainly the needle-punched holes and the pores between the fiber bundles in the weft-less layer,and the micro-pores are mainly present in the chopped fiber web and the fiber bundle in the weft-less layer.The needle process formed needle-punched holes with diameter of about 1 mm on the preform surface.On the macroscopic scale,with the increase of needle-punched density,the number and the overlapping of needle-punched holes gradually increased.The damage of fibers in the needle-punched site was aggravated.On the microscopic scale,the pore size of preform gradually decreased and the pore size distribution became more concentrated and uniform.(2)As the composites density increased,the total pore volume inside composite decreased gradually,and the decrease rate of total pore volume decreased.The pores were mainly large pores of 60-350?m and meso-pores of 10-60?m,the two pore volume of composites with different densities was all above 80%.With the increased of densification cycle,the volume of the large pores of 60-350?m decreased,the volume fraction of the micro-pores of 10-60?m and the micro-pore volume of less than 1?m increased gradually,and the volume fraction of pores about 1-10?m remained stable substantially.During the densification process,the resin-based carbon preferentially filled the macro-pores,the large pores in composites will change to meso-pores,and the meso-pores are partially will convert to small pores or micro-pores.(3)During the filling process,the open pores will evolve into closed pores,and the pore filling will evolve from a similar law.The pore evolution of the resin-based carbon composites conforming to the fractal features.The sponge fractal model was used to simulate the internal pore structure characteristics,and the complexity of the pore structure of the resin-based carbon composite was evaluated by the change of fractal dimension.As the density of composites increased,the porosity of composites gradually decreased,the fractal dimension of the pores became larger,and the structure of pores inside composites became more complicated.(4)When the needle-punched density of preform is 40 P/cm~2,comprehensive mechanical properties of composites is the best among the four parameters.Firstly,with the needle-punched densities increased in the range of 20~50 P/cm~2,the bending strengths of composites increased firstly and then decreased.It reached the highest value of 116.96 MPa when the needle-punched density is 40 P/cm~2.The bending failure characteristics of composites were brittle fracture,and the main bending failure mode was the bottom V-shaped failure.Secondly,the compressive failure characteristics of composites were plastic fracture.The transverse compressive strengths of composites were stabled and not followed the needle-punched density change.With the increased of the needle-punched densities,the longitudinal compressive strengths were gradually increased,the longitudinal compression failure modes were transformed from inter-layer delamination and shear failure to the end crushing damage.In addition,with the increased of densification cycle,the densification efficiency of composites decreased gradually,the transverse compressive strengths of composites increased,and the average strength increment ratio decreased gradually.As the densities of composites increased,the needle-punched densities of preforms have a greater effect on longitudinal compressive strength of composites.The increasingly complex pore structure adversely affected the compressive properties of resin-based carbon composites.